Long distance lightwave communication systems require amplifiers for boosting optical signal levels sufficiently to compensate losses experienced along the fiber transmission medium. Two classes of amplifiers are known, namely, lumped amplifiers and distributed amplifiers.
Lumped or discrete amplifiers are found in both semiconductor realizations and rare earth doped fiber embodiments. Rare earth doped fiber amplifiers have received a relatively high level of publicity in recent years because of their simplicity, low cost, and connective compatibility with existing optical fibers. For an exemplary locally pumped, rare-earth doped, fiber amplifier, see Electron. Lett., Vol. 23, No. 19, pp. 1026 et seq. (1987). In theory, these amplifiers linearly increase optical signal power of a supplied input signal via stimulated emission of fiber dopants such as Er.sup.3+ subject to a locally supplied optical pump source. While lumped amplifiers possess many beneficial attributes, it cannot be overlooked that their use adds noise such as amplified stimulated emission noise which accumulates from one amplification section to the next in a large communication system. Additionally, it is often necessary to include optical isolators or similar devices at the amplifier output to prevent unwanted feedback effects.
In response to the noise and feedback problems, distributed amplification systems based on stimulated Raman scattering and stimulated Brillouin scattering have been proposed. Such systems avoid the use of isolators and thereby operate bidirectionally. They provide distributed amplification on a substantially uniform basis which is an especially important characteristic for soliton-based lightwave systems. It is understood by those persons skilled in the art that these amplifiers also add amplified stimulated emission noise over the entire distributed amplifier. However, the amount of amplified stimulated emission noise is far less than that added by high gain lumped amplifiers. For reasonable pump powers on bidirectionally pumped fiber spans, it is expected that fiber spans are limited to be on the order of 50 km because the pump power exhibits exponential decay according to the loss coefficient of the fiber. Unfortunately, such short spans increase the cost of the lightwave transmission system with respect to both installation and maintenance.
At the present time, most telecommunication system designers specify long optical fiber spans for distributed amplification sections or between amplifiers on the order of at least 100 km. To achieve transmission over such long spans with cascaded lumped amplifier stages, it is possible to increase the optical signal power launched into the fiber at the transmitter to overcome the intrinsic loss of the longer optical fiber. However, such an approach causes significant signal intensity variations in the transmission fiber which lead to serious problems with nonlinear effects in the fiber itself and, possibly, to problems with saturation of the signal amplifiers. Nonlinear effects arising from nonuniform signal levels (i.e., a low signal level preceding an amplification stage and a very high signal level after amplification) are particularly deleterious when solitons are employed for signal transport. This is so because soliton-soliton collisions, which would normally be considered harmless for uniform amplification systems in that the effects of the approaching portion of the collision are substantially undone by the effects of the later departing portion of the collision, would now experience a large difference in nonlinear effects across the lumped amplifier causing collision effects to accumulate rather than cancel.
In response partially to the nonuniformity of signal amplification levels and to remedy the soliton-soliton collision problem experienced in lumped amplifiers, a distributed amplification technique has been disclosed in which bidirectional lightwave transmission is restored and uniform amplification of lightwave signals over long spans of optical fiber is achieved over potentially long spans. The disclosure is made in commonly assigned and copending application Ser. No. 418,000 (L. F. Mollenauer Case 14). Distributed uniform amplification is achieved by using an amplifying optical fiber which includes a long length of optical fiber having a dilute rare-earth dopant concentration substantially in the fiber core region, and a corresponding pump signal source at one or both ends of the doped fiber having the appropriate wavelength and power to cause amplification of optical signals by both Raman effects and stimulated emission from the rare-earth dopants. Dilute concentrations are understood as the range of concentrations substantially satisfying the condition that the gain from the rare-earth dopant, when pumped to nearly complete population inversion, is substantially equal to the fiber loss. While distributed uniform amplification is realized in one embodiment having a homogeneous span of optical fiber, other embodiments are shown in which distributed amplification is achieved using a combination of substantially long lengths (.gtoreq.1 km) of dilutely doped fibers together with long lengths of undoped fibers within the same span. Uniformly distributed amplification (gain) is achieved by a stimulated Raman effect in each undoped fiber and by stimulated emission in the doped fiber. One drawback to this approach for distributed amplification is the need to produce long lengths of a non-standard optical fiber product, namely, dilute rare earth doped silica fiber.